The emerald ash borer (Agrilus planipennis), also called EAB, is a green type of beetle from northeastern Asia. It feeds on ash trees (Fraxinus spp.). Female beetles lay eggs in cracks in the bark of ash trees. The larvae that hatch from these eggs live under the bark of ash trees and take one to two years to grow into adult beetles. In its home region, the beetle usually lives in small numbers and does not harm native trees. However, when it is not in its home region, it becomes an invasive species that causes serious harm to ash trees in Europe and North America. Before the beetle was discovered in North America, scientists knew little about it in its home region. This lack of knowledge has led to most research about the beetle’s life cycle being done in North America. Local governments in North America are trying to manage the beetle by tracking its movement, planting a variety of tree species, and using insecticides and biological control methods.
History
French priest and naturalist Armand David collected a specimen of the species during one of his travels through China in the 1860s and 1870s. He discovered the beetle in Beijing and sent it to France, where entomologist Léon Fairmaire published the first short description of Agrilus planipennis in the Revue d'Entomologie in 1888. Without knowledge of Fairmaire's earlier description, another scientist named Jan Obenberger published a separate description of the species as Agrilus marcopoli in 1930.
Identification
Adult beetles are usually bright metallic green and measure about 8.5 mm (0.33 in) in length and 1.6 mm (0.063 in) in width. Their elytra are often a darker green but may show copper colors. The emerald ash borer is the only Agrilus species in North America with a bright red upper abdomen when its wings and elytra are fully spread. This beetle has a small spine at the end of its abdomen and antennae with serrated edges starting from the fourth segment. It creates visible marks under the bark of damaged trees. Adult beetles from other species are frequently mistaken for this one by the public.
Life cycle
The life cycle of the emerald ash borer can take one or two years, depending on when the beetles lay their eggs, the health of the tree, and the temperature.
After 400–500 heat units above 10 °C (50 °F), adult beetles begin to emerge from trees in late spring. The most common time for adults to appear is around 1,000 heat units. Once they emerge, adults feed on ash leaves in the tree’s canopy for about one week before mating. This feeding causes little damage to the leaves. Male beetles fly around trees, find females using visual clues, and land on the female to mate. A mating session can last up to 50 minutes, and a female may mate with several males during her life. A typical female lives about six weeks and lays 40–70 eggs. Some females that live longer may lay up to 200 eggs.
Eggs are placed in bark cracks, flaky areas, or crevices and hatch in about two weeks. The eggs are about 0.6 to 1.0 mm (0.02 to 0.04 in) in size and start as white, turning reddish-brown if they are fertile. After hatching, the larvae chew through the bark to reach the inner layers of the tree, including the phloem, cambium, and outer xylem. They feed and grow in these areas. The larvae go through four stages, called instars. As they feed, they create long, winding tunnels. Fully grown fourth-instar larvae are 26 to 32 mm (1.0 to 1.3 in) long. In the fall, mature larvae dig chambers about 1.25 cm (0.49 in) deep into the sapwood or outer bark. They then curl into a J-shape and become prepupae. These prepupae turn into pupae and then adults the following spring. To leave the tree, adult beetles chew holes through the bark, creating a D-shaped exit hole. Some larvae may stay in their tunnels during winter but might need another summer of feeding before overwintering again and becoming adults the next spring. This two-year life cycle is more common in cooler areas, like European Russia.
- Larva
- Pupa removed from its pupal chamber
- Adults exit the tree from D-shaped holes.
- Dorsal view of adult with elytra and wings spread
- Underside of an adult emerald ash borer
Range
The emerald ash borer is originally found in the moderate climate regions of northeastern Asia, which include Russia, Mongolia, northern China, Japan, and Korea.
This beetle has become an invasive species in North America. Its main group is located in Michigan and nearby states and provinces. Outside this main area, the beetle is found in smaller, more scattered groups. The farthest known points of its spread in North America are north to Ontario, south to northern Louisiana, west to Nebraska, and east to New Brunswick. Additional groups have been found in Colorado and in Oregon in the Pacific Northwest.
In eastern Europe, the beetle was discovered in Moscow in 2003. Between 2003 and 2016, this group moved westward toward the European Union at a speed of up to 40 kilometers (25 miles) each year. Scientists predict this population will reach central Europe between 2031 and 2036. As of 2019, the beetle had not been recorded in the European Union, but it had already spread to far eastern Ukraine from Russia.
Host plants
In its natural habitat, the emerald ash borer is a minor pest that rarely harms healthy trees because its numbers are usually low. In China, it attacks native ash species such as Fraxinus chinensis, F. mandshurica, and F. rhynchophylla. In Japan, it also infests F. japonica and F. lanuginosa.
In North America, the emerald ash borer primarily targets ash species like green ash (F. pennsylvanica), black ash (F. nigra), white ash (F. americana), and blue ash (F. quadrangulata). In Europe, F. excelsior is the main ash species affected, and it has some ability to resist the pest. The susceptibility of ash trees to the emerald ash borer depends on factors such as the chemical signals that attract adult beetles or the ability of larvae to break down certain plant compounds. The beetle has been found infesting white fringe tree in North America, a non-ash species, though it is unclear if the trees were already unhealthy before being attacked. Another non-ash host, Olea europaea, has been observed in laboratory settings, but not in nature.
Adult beetles often lay eggs on ash trees that are stressed or growing in open areas, though they may also lay eggs on healthy trees among other species. Ash trees growing in pure stands—whether naturally or in landscaped areas—are more likely to be attacked than trees growing alone or in mixed forests. Landscaped ash trees often face greater stress from environmental factors like compacted soil, lack of water, urban heat, road salt, and pollution, which can weaken their ability to resist the pest. Most landscaped ash trees come from a limited number of cultivars, leading to low genetic diversity. Young trees with bark thickness between 1.5 mm (0.059 in) and 5 mm (0.20 in) are most commonly targeted. Both male and female beetles use chemical signals from leaves and bark to find host trees. Damage occurs when larvae feed inside the tree, creating winding tunnels that block the movement of water and nutrients. This blockage, called girdling, prevents the tree from transporting essential resources to its leaves, eventually killing it. Trees that are girdled may attempt to regrow from their stumps, and stressed trees may produce more seeds as a survival strategy.
Invasiveness
The emerald ash borer is an invasive species that causes serious harm to ash trees in areas where it is not native. Before it was discovered in North America, little was known about the insect in its home region, except for basic descriptions of its life cycle and classification. This lack of knowledge led to focused studies on its biology in North America. The insect was first found in Canton, Michigan, in 2002, but it may have arrived in the United States as early as the late 1980s. Scientists believe it was brought to the U.S. through shipping materials like packing crates from overseas.
In its native range, the emerald ash borer is kept in check by factors such as resistant trees, predators, and parasitoid wasps. Without these natural controls, its populations can grow rapidly and cause severe damage. After an infestation begins and no control measures are used, all ash trees in an area are expected to die within 10 years. Every type of ash tree in North America is vulnerable to the emerald ash borer, but some Asian ash species, such as F. baroniana, F. chinensis, F. floribunda, F. mandshurica, and F. platypoda, are resistant.
Green ash and black ash trees are most preferred by the emerald ash borer. White ash is also killed quickly, but usually only after green and black ash trees are gone. Blue ash shows greater resistance due to its high tannin content, which makes its leaves unappealing to the insect. While many Asian ash species have this defense, American ash species lack it except for blue ash. Researchers have studied "lingering ash" trees that survived attacks to develop more resistant tree varieties. Asian ash species also use natural defenses to repel, trap, and kill emerald ash borer larvae. Although American ash trees may have similar defenses, they do not seem to recognize when they are under attack.
Predators and parasitoids that control emerald ash borer populations in Asia are not present in North America. Native predators and parasitoids in North America do not effectively suppress the insect, allowing its populations to grow. Woodpeckers eat emerald ash borer larvae, but no native animals in North America eat the adult beetles. The insect spreads 2.5 to 20 kilometers (1.6 to 12.4 miles) per year through flight or by the movement of ash bark in products like firewood or nursery stock.
Cold winter temperatures, such as −38 °C (−36 °F), limit the insect’s range. Emerald ash borers can survive winter temperatures as low as −30 °C (−22 °F) due to antifreeze-like chemicals in their bodies and insulation from tree bark. Larvae can survive heat up to 53 °C (127 °F). The population of emerald ash borers in North America is believed to have originated from a single group of insects in central China and has low genetic diversity.
North American predators and parasitoids can sometimes kill many emerald ash borers, but they generally provide limited control. Mortality from woodpeckers varies, and parasitism by species like Atanycolus cappaerti can be high, but overall control remains low.
On December 14, 2020, the United States Department of Agriculture’s Animal and Plant Health Inspection Service ended all quarantine activities related to the emerald ash borer in the U.S. due to their limited effectiveness. Other methods, such as biological controls, are now being used.
The emerald ash borer has destroyed most of the 300,000 ash trees in the National Capital Region in nine years, leaving fewer than 80,000. Before the infestation, these forests had 17–18 ash trees per hectare, but now only 5–6 remain. This has disrupted ecosystems, with ash-dominated swamplands turning into shrublands and affecting wildlife.
The emerald ash borer threatens all ash trees in the North American genus Fraxinus. It has killed tens of millions of ash trees and is expected to kill most of the 8.7 billion ash trees across North America. The insect kills young trees before they reach their seeding age of 10 years. In both North America and Europe, the loss of ash trees can lead to more invasive plants, changes in soil nutrients, and impacts on species that rely on ash.
Efforts to control the emerald ash borer have affected businesses that sell ash trees or wood products, property owners, and local or state governments. Quarantines limit the movement of ash trees and products, but economic costs are high in urban and residential areas due to treatment or removal costs and reduced land value from dying trees. Managing these trees can be expensive for homeowners or local governments. Removing large numbers of dead or infested trees at once is costly, so slowing the rate of tree death by removing known infested trees and treating others with insecticides allows governments more time to plan, remove, and replace trees. This approach saves money, as removing and replacing all ash trees in 25 urban areas over 10 years would cost $25 billion, while a slower strategy would cost $10.7 billion. Some cities, like Minneapolis, have large populations of ash trees, with slightly more than 20% of their urban forest being ash.
In areas where the emerald ash borer has not yet been found, surveys are used to detect new infestations. Visual surveys look for ash trees showing signs of damage, and traps with colors like purple or green are placed in trees as part of monitoring programs. These traps may also use pheromones to attract male beetles.
Sometimes, trees are girdled to act as trap trees. The stressed tree attracts egg-laying females in the spring, and the tree can be debarked in the fall to search for larvae. If the insect is detected, an area is often placed under quarantine to prevent the spread of infested wood. Additional control measures are then taken to reduce beetle numbers, prevent them from reaching reproductive maturity, and decrease the number of ash trees.
Government agencies in the U.S. and Canada have used a native parasitoid wasp, Cerceris fumipennis, to detect emerald ash borer infestations. These wasps hunt jewel beetles and emerald ash borers if present. They stun the beetles and carry them to their burrows, where their eggs hatch and larvae feed on the beetles. Volunteers catch the wasps as they return to their burrows to check if emerald ash borers are present. This method is called biological surveillance, not biological control, because the wasps do not significantly reduce emerald ash borer populations.
In areas where the emerald ash borer is invasive, quarantines, removal of infested trees, insecticides, and biological control are used to protect ash trees. Once an infestation is detected, quarantines are usually imposed by state or local governments.